1. Field of the invention:
The present invention relates to a display apparatus for a performing display by applying a drive signal to displaying picture element electrodes by means of switching elements. More particularly, it relates to an active matrix drive mode display apparatus which disposes the picture element electrodes in a matrix so as to perform high density display.
2. Description of the prior art:
A liquid crystal display apparatus, an electro-luminance display apparatus and a plasma display apparatus have hitherto been selected to display picture elements disposed in a matrix form to form a display pattern on a picture plane. A method for selecting display picture elements includes an active matrix driving method which disposes individually independent picture element electrodes and connects the switching element to the respective picture element electrodes for display drive. This method enables displays in high contrast and is put in practical use for liquid crystal television, word processors, terminal display units for computers, or the like. The switching element for selectively driving the picture element electrodes is either a thin film transistor (TFT) element, a metal-insulator-metal (MIM) element, a MOS transistor element, a diode, a varistor or the like. Voltage applied between the picture element electrodes and an electrode apposite thereto is switched to optically modulate a display medium, such as liquid crystal, EL light emission layer or plasma luminosity, the optical modulation being visually recognized as the display pattern.
When the switching elements are connected to the picture element electrodes for carrying out the high density display, it is required to dispose a great many picture element electrodes and switching elements. The switching element, however, may be a malfunctioning element at the time when it is packaged on a substrate, and the picture element electrode connected to such a poor element leads to a picture element defect that dose not contribute to the display.
The technique for restoring the picture element defect has been disclosed in, for example, Japanese Laid-Open Patent Publication No. 61-153619, in which a plurality of switching elements are provided per one picture element electrode, and only one of these switching elements is connected to the picture element electrode. The switching element connected to the picture element electrode, when it is poor, is cut off from the picture element electrode by a laser trimmer, an ultrasonic cutter or the like, and another switching element is connected to the picture element electrode. In this case, the switching element and picture element electrode are connected therebetween by bonding a minute conductor with a dispenser or the like, or by coating Au, Al or the like at a predetermined location on the substrate. Furthermore, Japanese Laid-Open Patent Publications No. 61-56382 and No. 59-101693 disclose the technique for irradiating the laser light to melt metal so as to electrically connect between the metal layers.
The above-mentioned conventional techniques for restoring the defect, after detection thereof, irradiate the laser light to evaporate and redeposite metal or locally melt the metal, thereby performing an electrical connection. In other words, these techniques must be used in the manufacturing process for active matrix substrates prior to the assembly of the display panel. The reason for this is that, after completion of the display panel, part of the metal evaporated or melted by irradiation of laser light is mixed into the display medium such as liquid crystal, which is interposed between the picture element electrode and the opposite electrode thereto, and thereby remarkably deteriorates the optical characteristics of the display medium. Accordingly, both the above-mentioned conventional methods for restoring picture element defects are applied to an active matrix substrate manufacturing process prior to the display panel assembly, in other words, before the display medium is charged.
However, it is very difficult to detect the picture element defect in the process of manufacturing the active matrix substrate. Especially, for a large-sized panel of picture elements of one hundred thousand to five hundred thousands or more, measurement equipment of extremely high accuracy must be used to detect the electrical characteristics of all the picture element electrodes so as to find a poor switching element. As a result, the detection process becomes complicated, the mass productivity is impeded, and the display apparatus has a high production cost. Accordingly, the fact is that the aforesaid restoring techniques cannot be used for the large-sized display panel with a large number of picture elements.